1. Field of the Invention
The present invention relates to an apparatus for the optical detection of flaws in articles as they are being knitted in a circular knitting machine, and in particular to an apparatus for detecting flaws in footwear, irrespective of whether such flaws arise from faults in the knitting process or from various other origins e.g. the presence of yarns that are too thick or too thin, or from faults in the plaiting.
2. Prior Art
Flaw detection systems for fabrics based on optical inspection have already been proposed in the prior art. Such systems are described e.g. the publication "L'Industrie Textile" No. 1067 pages 277 to 279. They comprises a luminous source, an electro-optical sensor receiving light from the optical source after it has either been reflected from or passed through, the fabric, and means for processing data from the electro-optical sensor. The fabric is laid out flat and made to pass in front of tubular fluorescent lamps powered by a DC voltage source in order to illuminate the fabric throughout its width. Electro-optical cells are aligned along a row and extend throughout the width of the fabric. The lamps and the cells are either both on the same side of the fabric or on either side. In the first case, the cells measure the light reflected by the fabric. In the second case they measure the light passing through the fabric. Any change in the fabric's structure or surface condition resulting from flaws in the weaving or in the dressing brings about a change in the current of the photoelectric cell concerned, so triggering a signal which is amplified by an amplifier and sent to an analog-digital converter for subsequent processing. By using adjustable thresholds it is possible to set the level of the signal that qualifies as a flaw indication.
The flaw inspection systems for fabrics described in the above article are not readily amenable to the detection of flaws in articles being knitted in a circular knitting machine and even less so in the case of knitting machines having a small diameter, as used in the manufacture of items of footwear. The reason is that in such machines, the knitted fabric is produced by a rotation of the knitting head and is then passed around a mandrel such that the part of the knitted fabric open to inspection is very difficult to access.